<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
        <div class="parent">
            <div class="test"></div>
        </div>
</body>
</html>
<style>
.parent{
    width: 100px;
    height: 100px;
    border: 1px solid #333;
    /* 第一种 */
    /* position: relative; */
    /* 第二种 */
    /* display: flex;
    justify-content: center;
    align-items: center; */
}
.test{
    width: 40px;
    height: 40px;
    background-color: pink;
    /* 第一种 */
   /* position: absolute;
   left: 50%;
   top: 50%;
   transform: translate(-50%,-50%); */

    /* 第三种 */
   margin-left: 50%;
   margin-top: 50%;
   transform: translate(-50%,-50%);
}
</style>